Field of the Invention
The present invention relates generally to automatic implantable cardioverter/defibrillators and implantable programmable pacemakers, and more particularly to a system which combines a non-programmable automatic implantable cardioverter/defibrillator with an implantable programmable pacemaker in a way which allows the pacemaker to programmably control, at least in part, the operation of the otherwise non-programmable cardioverter/ defibrillator.
An automatic implantable cardioverter/defibrillator (hereafter "AICD") provides one or more high energy stimulation pulses to a heart when: (1) the AICD senses that the heart is beating too fast (tachycardia); or (2) the AICD senses that the heart is not beating at all (fibrillation). In the case of a tachycardia, the delivery of the high energy stimulation pulse or pulses is usually referred to as "cardioversion", and the stimulus is delivered in synchrony with the heart's QRS wave. This synchronization is done to avoid stimulating the heart during the T-wave portion of the P-QRS-T cardiac cycle.
In the case of fibrillation, the heart has stopped so there is no QRS complex. Hence, there is no need to synchronize the delivered high energy stimulation pulse or pulses with any cardiac event. The purpose of delivering the high energy stimulation pulse or pulses to the heart is, of course, to shock the heart back into beating at a more normal rate, i.e., to break the tachycardia, or to defibrillate the fibrillating heart. The high energy stimulation pulses generated by the AICD are commonly referred to as either "cardioversion" pulses or "defibrillation" pulses.
Conventional AICD devices known in the art typically include a built-in sensor circuit for sensing, through attached sensing electrodes, the rate at which the heart is beating. If the sensed heart rate exceeds a high fixed rate threshold, the AICD is triggered to deliver a cardioversion pulse to the heart through separate stimulating electrodes. If the sensed heart rate is less than a low fixed rate threshold, the AICD is triggered to deliver a defibrillation pulse to the heart through the stimulating electrodes. By using fixed, non-programmable, rate thresholds in this fashion, the sensing and stimulating circuits of the AICD device may be kept simple and compact.
The doctor or physician who is implanting an AICD device must determine the correct high and low fixed rate thresholds which are needed for the particular patient. Typically, only the high rate threshold is of concern, because the low rate threshold is usually just a long escape interval which must time out without any cardiac activity being sensed. However, for some patients exhibiting bradycardia, even the low rate threshold must be carefully selected.
Once the correct thresholds are determined, the doctor must obtain an AICD device which has the desired thresholds built-in to its design. Unfortunately, this requires that the AICD device be specially ordered from a manufacturer, or that the implanting doctor maintain a large inventory of AICD devices covering a wide threshold range. Further, after implant, there is some likelihood that the correct high rate threshold for the patient may change, particularly if the patient is taking prescribed medication (drugs). Hence, there is a need in the art for a programmable AICD device wherein the high and low rate thresholds may be readily modified.
Moreover, in view of the already existing AICD devices having fixed high and low rate thresholds, there is an immediate need in the art for a programmable means or system for modifying, as required, the fixed rate thresholds associated with these devices. The present invention advantageously addresses these and other needs. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.